The term "four-phase modulation" is used in the telecommunication art to describe a technique by which a sinusoidal carrier wave is selectively shifted into any of four possible phase positions relative to a reference wave of the same or a subharmonically related frequency. These phase positions represent respective pairs of consecutive bits of a data word, known as dibits, which can be decoded at a receiver by coherent phase demodulation involving a comparison with a reference wave in step with that used at the transmitter. The phase-modulated carrier can be regarded as the sum of a cosine wave and a sine wave of like frequency and of amplitudes of the same unit value .+-.1; by combining a cosine wave in phase with the reference wave and a sine wave in quadrature therewith, the resulting carrier will be phase-shifted by selective inversions of the sign of either or both constituent waves. Thus, the logical values of the first and second bits of a dibit may be respectively represented by the signs of the cosine component and the sine component, for example; conventionally, phase positions in the first, second, third and fourth quadrants (45.degree., 135.degree., 225.degree. and 315.degree.) may denote the bit pairs 11, 10, 00 and 01, respectively. The actual synthesis of the carrier at the transmitting end may be performed digitally with the aid of predetermined filter coefficients by which instantaneous samples of the cosine and sine components are logically multiplied before being added together; a circuit arrangement performing these operations is disclosed in commonly owned U.S. application Ser. No. 100,908, filed Dec. 6, 1979 in the names of Francesco Gandini et al as a continuation of an earlier application, Ser. No. 902,164, filed May 2, 1978.
The use of four-phase modulation is particularly advantageous in a telephone system in which it is desired to transmit data at high speed among participating subscribers, e.g. digital speech at 64 Kbit/sec., signalization and synchronization messages at 16 Kbit/sec., or slow video signals. Full-duplex data transmission over a two-way signal path is possible with the aid of two carriers of different frequencies. The need may occasionally arise for interchanging the transmitting and receiving frequencies since the high-frequency channel is subjected to greater attenuation whereas the lower frequency channel may be affected by a more pronounced amplitude dissymmetry. Also, a given terminal may receive carriers of higher and lower frequency from different signal links.